Laser light has a high level of coherence and therefore tends to cause speckle noise. Speckle noise may cause problems in various uses of laser light sources. Recently, display devices using semiconductor lasers as light sources have been considered. In these display devices, display quality may be degraded by speckle noise. Therefore, there is demand of speckle noise reduction.
Meanwhile, to reduce laser noise resulting from return light, a technique of driving a semiconductor laser by a drive current composed of a direct current and a high-frequency current superposed thereon is known.